Thursday, July 8, 2010

Accessibility: Intermediate-Advanced

How is letter processing different from word processing? Since letters compose words, many reading models have letter processing earlier in the reading stream, but there is still room for more imaging work.

Turkeltaub and colleagues compared the neural basis of letter processing in children (age 6-11) and adults (age 20-22). The participants were scanned while naming either letters or line drawings out loud. Here are four of their findings.

1. Adults had more activation than children in visual regions. This appeared to be driven mostly by differences in letter naming*. This suggests that object processing might be more adult-like in kids at this age.

2. Areas showing a change in letter processing with age were posterior to regions found in other studies to respond to words. Since visual processing moves from back to front, this fits with a model in which letters are processed before words.

3. The authors found no left hemisphere dominance for letters. This very different words, which are heavily left lateralized. This is also different from Cantlon 2010 which did find letter processing to be left lateralized. I wonder if the results here could be different if the authors had used another method to pick their analysis region**.

4. The authors also found that no regions activated more for letters than for objects. This is consistent with what I also find in my data. Objects are more visually complex than letters, so it's not surprising that you get more activation for objects. I should note that Cantlon found regions that responded more to letters than objects, but Cantlon only used shoes, which as a set are more uniform than line drawings of different objects.

*although there is no interaction between objects and letter naming

**ROI selection based on activation for all tasks.

Turkeltaub PE, Flowers DL, Lyon LG, & Eden GF (2008). Development of ventral stream representations for single letters. Annals of the New York Academy of Sciences, 1145, 13-29 PMID: 19076386